For some applications it is useful to image a scene from two angles to obtain a stereoscopic viewing effect or for other reasons. For example, an imaging system in an earth-orbiting satellite spacecraft may include a first telescope aimed toward the scene at a first angle, and a second telescope aimed toward the scene at a second angle. The images from the two telescopes may be used to form a stereoscopic image of the scene, if the two telescopes are aimed at the proper angles. The images may also be used to provide information that a single image cannot, even if not arranged for stereoscopic imaging. For example, a satellite flying from south to north (the scan direction) may use a forward-inclined telescope to image the south side and then the top of a building before the satellite passes over the building, and a rearward-inclined telescope to image the top and then the north side of the building after the satellite passes over the building.
Each telescope includes an optical train with reflective and/or refractive optics that gathers the light from the scene and images it at a focal surface. Each telescope includes a sensor such as a focal plane array positioned at the focal surface to receive the image from the optical train and convert the image to an electrical signal for signal processing.
It is desirable that the telescopes have a large field of view, in order to gather the greatest amount of information in each pass over the earth's surface. To obtain a large field of view with each telescope, very large two-dimensional focal plane arrays have been developed. Such large focal plane arrays may have 5000 or more pixels in each dimension, so that there are tens to hundreds of millions of total pixels in each focal plane array. The large focal plane arrays produce a large field of view of at least several degrees in the cross-scan direction. The large focal plane arrays also allow the image exposures to occur relatively infrequently, such as every second or so, as the satellite moves along its track in the scan direction.
Both the optical train and the sensor are expensive components. Additionally, the two telescopes must be positioned with sufficient clearance from each other so that they do not interfere, resulting in a relatively large size spatial envelope. There is a need to reduce the costs of the imaging system and, if possible, its spatial envelope size to permit a reduction in size and weight of the spacecraft at launch. The present invention fulfills this need, and further provides related advantages.